Fire is one of the most important innovations in humanity's evolutionary history, but it's also one of the most mysterious. It leaves almost no trace in the archaeological record, and it's often impossible to determine when humans began controlling fire.

As such, the range of dates for when humans - or our evolutionary ancestors - first turned fire from a natural phenomenon to a tool is ridiculously large, spanning anywhere from 1.7 million years ago to as recently as 400,000. Both of these times long predate the emergence of Homo sapiens about 200,000 years ago, but they represent the extreme endpoints of the existence of Homo erectus, the hominin species that most likely was the first to wield the flame.

That's why a new discovery in South Africa's Wonderwerk Cave is so crucial. A massive cave on the edge of the Kalahari Desert, Wonderwerk was inhabited by hominins for hundreds of thousands of years. A newly excavated layer dating back about one million years ago reveals clear evidence of ashed plants and burned bones. These microscopic traces were found alongside those of stone tools, suggesting a link with the cave's hominin occupants.

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What's more, the artifacts were found deep enough in the cave that the fire was almost certainly started intentionally, rather than carried into the cave by wind or water. The surrounding surfaces from that time period also showed signs of discoloration that are typical of frequent fires. All the evidence suggests the hominins that occupied this cave a million years ago used fire.

Whether they had actually mastered it is another question. The lead excavator, Michael Chazan of the University of Toronto, think it's more likely that the Homo erectus of Wonderwerk simply happened upon wildfires and carried them inside the cave, making opportunistic use of the fire rather than actively controlling it. Even so, Chazan speculates that these still essentially accidental fires could have had a major impact on the residents of Wonderwerk and hominins in similar circumstances elsewhere:

"The control of fire would have been a major turning point in human evolution. The impact of cooking food is well documented, but the impact of control over fire would have touched all elements of human society. Socializing around a camp fire might actually be an essential aspect of what makes us human."

The confirmed use of fire at least a million years ago is a big deal, but it's still a far cry from hypotheses that call for the use of fire as early as 1.9 million years ago. Favored by Harvard's Richard Wrangham, this idea holds that fire and cooking foods actually helped reshape our evolutionary path by freeing up energy that could be used to sustain bigger brains. While Wonderwerk pushes back the confirmed timeline, it doesn't exactly help the hypothesis, considering all the new evidence argues for opportunistic, occasional use of fire, not the sort of thing likely to jumpstart major physiological adaptations.